Flour doesn't turn brown when kept in the refrigerator for a long time because browning, or Maillard reaction, requires certain conditions that are typically not present in the cold environment of a refrigerator.
The Maillard reaction is a chemical reaction that occurs between amino acids (found in proteins) and reducing sugars when exposed to heat. This reaction is responsible for the browning and development of complex flavors in various cooked foods, such as bread crusts, roasted meats, and baked goods.
In the refrigerator, the temperature is low, typically around 4°C (40°F) or below. At such low temperatures, the Maillard reaction does not occur or progresses extremely slowly. The reaction requires higher temperatures, usually above 120°C (248°F), which are typically achieved during cooking or baking.
Additionally, the refrigerator helps to protect flour from exposure to air and moisture, which are also essential factors for the Maillard reaction to take place. When flour is stored in a cool, dry, and airtight environment like a refrigerator, it remains relatively stable and retains its original color.
However, it's worth noting that prolonged storage of flour, even in the refrigerator, can still lead to changes in quality over time. Flour can absorb odors from the refrigerator and other nearby foods, affecting its flavor. Therefore, it's best to store flour in an airtight container in a cool, dry pantry or cupboard for the best results and to use it within a reasonable time frame to maintain its freshness and quality.